1. Technical Field
The present invention relates to area light source devices, and in particular, to an area light source device used in a back light or the like to illuminate a liquid crystal panel from a rear surface.
2. Related Art
In an edge light type area light source device, a light source is arranged facing a light incident end face of a flat plate shaped light guide plate, and a diffuser plate or a prism sheet is overlapped on the upper surface of the light guide plate. In the area light source device, if the thickness of the light guide plate is smaller than the height of the light source, the light emitted from the light source does not enter the light guide plate from the light incident end face and the light that escapes to the outer side than the light incident end face increases, and hence the usage efficiency of the light worsens. The thickness of the light guide plate is thus made substantially equal to the height of the light source.
Consideration is made in thinning the height of the light source and the thickness of the light guide plate when attempting to thin the entire thickness of the area light source device. However, there is a restriction in manufacturing for the light source and it is difficult to reduce the height even if the thickness of the light guide plate can be thinned. Therefore, the thickness of the area light source device cannot be thinned to smaller than or equal to a certain extent with the method of reducing the height of the light source and the thickness of the light guide plate.
As shown in FIG. 1, an area light source device 11 in which a thickness of a light incident unit 15 including a light incident end face 14 is substantially equal to the height of a light source 12, a flat plate shaped light guide plate main body 17 having a thickness thinner than the light incident end face 14 is arranged on the side opposite to the light incident end face 14 of a light guide plate 13, and a wedged shaped light introducing unit 16 is formed between the light incident unit 15 and the light guide plate main body 17 so as to continue them is proposed. In the area light source device 11, the difference between the thickness of the light incident end face 14 and the thickness of the light guide plate main body 17 is made substantially equal to the total thickness of a diffuser plate 19 and prism sheets 20, 21, so that the entire thickness of the area light source device 11 can be thinned by overlapping the diffuse plate 19 and the prism sheets 20, 21 on the upper surface of the light guide plate main body 17. Furthermore, in the area light source device 11, the light emitted from the light source 12 is efficiently retrieved into the light incident unit 15 from the light incident end face 14 since the height of the light source 12 and the thickness of the light incident end face 14 are substantially equal. Moreover, the light that entered the light incident unit 15 is guided to the light guide plate main body 17 while totally reflecting at an inclined surface 18 and a lower surface of the light introducing unit 16 so that the leakage light at the light guide plate 13 can be reduced since the upper surface of the light introducing unit 16 is the inclined surface 18 and the light incident unit 15 having a large thickness and the light guide plate main body 17 having a thin thickness are smoothly connected through the light introducing unit 16.
However, if the area light source device is used as a back light of the liquid crystal panel, the upper surface of the light introducing unit and the outer peripheral part of the upper surface of the light guide plate main body are covered with a light shielding tape (mainly black adhesive sheet) and the liquid crystal panel is overlapped on the upper surface thereof to prevent stray light from entering the liquid crystal panel. The light shielding tape thus sometimes sticks to the inclined surface of the light introducing unit, where the light in the light guide plate entered from the inclined surface is absorbed by the light shielding tape when the light shielding tape sticks to the inclined surface, and hence the luminance of the area light source device may lower or luminance unevenness may occur.
In the area light source device disclosed in Japanese Patent Publication No. 4279761, the end of a diffuser plate 19 positioned at the very bottom of the sheets overlapped on the upper surface of the light guide plate main body 17 is extended to be overlapped on the upper surface of the inclined surface 18, so that the extended diffuser plate 19 prevents the light shielding tape 19 from sticking to the inclined surface 18, as shown in FIG. 2.